1. Field of the Invention
The invention relates to wireless communication systems, and more particularly to data transmission of wireless communication systems.
2. Description of the Related Art
A discontinuous reception (DRX) mode is an essential operation for saving energy in the 3GPP specification. A wireless communication system comprises a base station and user equipment. The user equipment is often a handheld device having a limited power supply, and reduction of power consumption of the user equipment is therefore an important issue to extend the battery life of the user equipment. FIG. 1 shows a schematic diagram of signal transmission in a discontinuous reception mode. A period is sliced into a series of periodic data transmission cycles. When the wireless communication system operates in the discontinuous reception mode, the base station only sends control signaling to the user equipment during active periods of the data transmission cycle. The user equipment, therefore, only needs to receive the control signaling during the active periods, and can enter a sleeping state with low power consumption when not in the active periods. The power consumption of the user equipment is therefore reduced to extend the battery life of the user equipment. A wireless communication system operating in a semi-persistent scheduling (SPS) mode has similar communication operations with a wireless communication system operating in the discontinuous reception mode.
A base station operating in a discontinuous reception mode transmits control data to user equipment via a plurality of the component carriers with different frequency bands. Each component carrier may have different active periods. According to the relationship between the active periods of different component carriers, the base station may send control signaling to the user equipment with three different methods. FIG. 2A shows the first schematic diagram of active periods of three component carriers according to an identical configuration method is shown. According to the identical configuration method, the active periods of the three component carriers are the same. A base station may send control signaling to the user equipment during the same active periods via the three component carriers. The active periods of the three component carriers therefore have the same configuration.
FIG. 2B shows the second schematic diagram of active periods of three component carriers according to an independent configuration method. According to the independent configuration method, the three component carriers have different active periods and the DRX cycle. A base station therefore sends control signaling to the user equipment during different active periods via the three component carriers. Because the active periods of the three component carriers have different configurations, the base station must send an increased amount of the configuration messages to the user equipment to synchronize transmission and reception of the control signaling. FIG. 2C shows the third schematic diagram of active periods of three component carriers according to an anchor carrier based method. According to the anchor carrier based method, a carrier is determined as an anchor carrier. A base station mainly sends control signaling to the user equipment via the anchor carrier during active periods. When the base station uses other component carriers, the base station sends a notification message to the user equipment via the anchor carrier in advance and user equipment receive control signaling on the activated carriers.
All the aforementioned three methods have shortcomings. According to the identical configuration method, the user equipment must comprise multiple radio frequency modules to respectively receive radio signals via the multiple component carriers at the same active time periods, and therefore consumes more power. According to the independent configuration method, the active periods of the three component carriers are different, and the base station must send extra configuration messages to synchronize transmission and reception of the control signaling. According to the anchor carrier based method, only the anchor carrier is used to transmit control signaling and other component carriers are left idle without transmitting control signaling. Thus, all of the identical configuration method, the independent configuration method, and the anchor carrier based method have deficiencies. A new method for arranging active periods of different component carriers of a wireless communication system operating in a discontinuous reception mode is therefore required to improve the performance of the wireless communication system.